1. Field of the Invention
The present invention relates to a digital subscriber system, and more particularly, to a digital subscriber line signal receiving apparatus unsusceptible to HAM radio interference.
2. Description of the Related Art
Demand for wideband multimedia services is increasing due to the popularity growth of video on demand (VOD), interactive video, teleconferencing, browsing the internet, and downloading large audio and video files from the internet. Optimally, wideband multimedia services are supplied to the subscriber via wideband techniques, so data will flow through the network to the subscriber without delay. Asynchronous transfer mode (ATM) networks typically carry this type of traffic, so an optical cable must be installed from the ATM network to the subscriber""s home to prevent a data flow bottleneck leaving the ATM network. However, optical cable installation is considerably expensive and time consuming. A less costly and more quickly installed alternative is digital subscriber line (DSL or XDSL) since it can utilize an existing local loop within a telephone network. xDSL more specifically refers to high-bit-rate DSL (HDSL), asymmetric DSL (ADSL), very-high-bit-rate DSL (VDSL), etc. A typical high speed DSL end to end connection has a local loop connected to the connection originator and a local loop connected to the connection destination. An optical network such as ATM can connect the two local loops. DSL technology mixes high speed data with general telephone service over an existing telephone line to the subscriber. Typically, multimedia applications need high bandwidth going from the data source to the subscriber. For example, a VOD session would stream compressed video data (a huge amount of data) from an information source to the subscriber and control data (e.g. video selection) from the subscriber to the information source.
VOD service is an asymmetrical service because the download channel from a video server to a subscriber has much larger bandwidth than the upload channel from the subscriber to the video server. An asymmetric digital subscriber line (ADSL) is used with respect to the asymmetrical service. ADSL technology uses a local loop to connect the subscriber to a central office (CO) or an optical network unit (ONU). As the traffic speed or bandwidth increase the local loop distance must decrease. However, a repeater may be placed in the local loop to increase the minimum distance from the CO or ONU to the subscriber. The local loop uses an existing copper wire subscriber line, so equipment and service installation are as simple as for conventional telephone service.
Various signals are simultaneously transmitted using a pair of telephone lines without overlapping frequency bands by locating video and control signal bands above the frequency band used by a conventional telephone signal.
The most severe obstacle in DSL is radio frequency interference (RFI) signals due to HAM radio. The RFI is mainly generated in a portion of the network in which a drop cable for distributing a signal is connected to a lower cable.
FIG. 1 illustrates the transmission setup of a general digital subscriber line. A transmitting end 11 of a digital subscriber line transmits data from an information source in the direction of a subscriber. A data signal transmitted by the transmitting end 11 passes through a communication channel 12 including a transmission line. The transmission line is copper, so interference noise 13 and an RFI signal 14 such as narrowband noise or white Gaussian noise are added to the signal resulting in a signal received by a receiving end 15.
FIG. 2 shows the receiving end 15 of FIG. 1 in detail. When a receiving end system is turned on, an initialization controller 28 sequentially initializes an automatic gain controller (AGC) 20, timing recoverer 22, and an equalizer 24 respectivelly. The AGC 20 amplifies the received, signal to a predetermined signal size. The timing recoverer 22 searches the signal output by the AGC 20 for correct timing information thereby generating a synchronized signal. The equalizer 24 removes noise from the synchronized signal and outputs the resulting signal. A slicer 25 compares the level of the signal output from the equalizer 24 to a predetermined level to convert the received signal to a digital signal. Additionally, a signal to noise ratio (SNR) sensor 30 compares the signal output by the equalizer 24 to a predetermined SNR value and informs the initialization controller of the result.
When a HAM radio signal is applied to the signal received by the receiving apparatus shown in FIG. 2 or the HAM radio signal is applied and then disappears, the equalizer 24 of the receiving end outputs an erroneous signal which suddenly increases. Hence, a reduction in the signal to noise ratio (SNR) of the output signal of the receiving apparatus. The SNR sensor 30 informs the initialization controller 28 of the change in SNR, so the initialization controller 28 re-initializes the AGC 20, the timing recoverer 22, and the equalizer 24.
The digital subscriber line receiving apparatus according to the conventional technology repeats such an initialization process whenever the HAM radio signal is applied or the HAM radio signal is applied and disappears. Accordingly, data is lost during the transmission and reception of data. Therefore, smooth transmission and reception of data cannot be performed.
The present invention overcomes the problems of the conventional technology by being resilient to HAM radio interference. It is an feature of the present invention to provide an apparatus for receiving a digital subscriber line signal which is free from HAM radio interference noise for preventing data from being lost due to the characteristic of a HAM radio signal included in a signal received by a digital subscriber receiving apparatus.
It is another feature of the present invention to provide a method for receiving the digital subscriber line signal.
Accordingly, to achieve the first objective, there is provided a digital subscriber line receiving end apparatus resilient to HAM radio interference noise, for receiving a signal from an information source through a communication channel, comprising:
an automatic gain controller for amplifying a received signal to a predetermined magnitude;
a timing recoverer for recovering timing from a signal output from the automatic gain controller;
an equalizer for removing interference noise from a signal output by the timing recoverer;
an initialization controller for resetting and initializing the automatic gain controller, the timing recoverer, and the equalizer; and
an update controller for comparing signal to noise ratio of a signal output by the equalizer to a reference signal to noise ratio, and freezing a re-initialization state of the automatic gain controller and the equalizer for a predetermined length of time when the signal to noise ratio of the signal from said equalizer is greater than the reference signal to noise ratio and HAM radio interference noise is detected in the signal from said equalizer.
To achieve the second feature, there is provided a method for recovering a signal having HAM radio interference noise in a digital subscriber line receiving end apparatus, comprising the steps of (a)initializing a automatic gain controller, a timing recoverer, and a equalizer for a predetermined period of time, (b) determining the existence of HAM radio interference noise by comparing a signal to noise ratio of an output signal from the equalizer to a reference signal to noise ratio, (c) continuing step (b) after initializing the automatic gain controller, the timing recoverer, and the equalizer again when HAM radio interference noise is not detected in step (b), and (d)continuing step (b) after initializing the automatic gain controller, the timing recoverer, and the equalizer again and freezing the automatic gain controller and the equalizer for a predetermined time, when the HAM radio interference noise is detected in step (b).